1. Field of the Invention
The present invention relates to a charged particle microscope which is preferably applied to an electron microscope such as an environment-control-oriented-scan type electron microscope having a low vacuum sample chamber.
2. Related Background Art
An electron microscope is being used to observe or to measure the length of fine patterns formed on a sample. As such an electron microscope, an environment-control-oriented-scan type electron microscope has been recently developed, which enables observation and length measurement of a sample put in a low vacuum sample chamber.
FIG. 2 shows a conventional environment-control-oriented-scan type electron microscope. In FIG. 2, a sample 2 is contained in a sample chamber 1, and a first electrode 4 is attached through an insulating material 3 on the upper part of the inner wall of the sample chamber 1. Though schematically shown in FIG. 2, the first electrode 4 has a ringed shape whose axis coincides with the optical axis of an electronic optical system. A high vacuum chamber 6 is communicated with the sample chamber 1 through an aperture, and a charged particle irradiation means 7 is provided in the high vacuum chamber 6. The surface of the sample 2 is scanned with a primary electron beam 8 emitted from the charged particle irradiation means of the electron microscope.
Gas (for example, water vapour) which is used for gas amplification is charged from an external gas entrapping unit (not shown) through a gas charge valve 9 into the sample chamber 1. The same gas used for gas amplification is discharged out of the sample chamber 1 through a gas exhaust valve 10. The pressure of the gas inside the sample chamber 1 is detected by a pressure sensor 11. The result of this detection is applied to a pressure controller 12, which controls the gas charge valve 9 and the gas exhaust valve 10. Thus, the pressure of the gas inside the sample chamber 1 is kept within a range from 0.01 to 20 Torr, which is externally instructed.
The gas inside the sample chamber 1 also flows into the high vacuum chamber 6 through the aperture communicating with both chambers. So, in order to keep the pressure of the gas in the sample chamber 1 used for gas amplification at a predetermined value, the gas inside the high vacuum chamber 6 is constantly discharged by a vacuum pump (not shown). Such a mechanism is, however, indeed very complicated. Therefore, for brevity, FIG. 2 shows the system as if pressure control were performed only by the pressure controller 12.
The first electrode 4 is connected with a signal current input terminal 14a of a sample picture signal amplifier 13. The sample picture signal amplifier 13 consists of an operation amplifier 14 and a resistor 15. The signal current input terminal 14a is connected with the inverting input terminal of the operational amplifier 14 and an end of a the resistor 15. The non-inverting input terminal 14b of the operational amplifier 14 is connected with a floating ground terminal 16b of a first high voltage power source 16. The positive power source input terminal 14p of the operational amplifier 14 is connected with the positive power source output terminal 16p of the high voltage power source 16, while the negative power source input terminal 14n of the operational amplifier 14 is connected with the negative power source output terminal 16n. Thus, a sample picture signal S1 is generated at the output terminal of the operational amplifier 14 connected with the other end of the resistor 15.
In this constitution, the potential of the floating ground terminal 16b of the first high voltage power source 16 is controlled to be a direct current voltage E1, which is within a range from 0 to 600 V, by an externally applied signal voltage V1. In addition, the potential of the non-inverting input terminal 14b of the operational amplifier 14 is equal to the potential of the inverting input terminal. Accordingly, said direct corrent voltage E1 is applied to the first electrode 4.
On the other hand, a voltage of (E1+12)[V] is output from the positive power source output terminal 16p of the high voltage power source 16, while voltage of (E1-12)[V] is output from the negative power source output terminal 16n of the high voltage power source 16. That is, the sample picture signal amplifier 13 is an amplifier, to which two kinds of power having a difference of .+-.12[V] are applied, of current-input/voltage-output type. The overall potential of the sample picture signal amplifier 13 is increased up to the direct current voltage E1 by the externally applied signal voltage V1.
Now, suppose the voltage V1 is set to be, for example, 400[V]. When the charge particle irradiation means 7 of the electron microscope irradiates the sample 2 with the primary electron beam 8, secondary electrons 18 generated from the sample 2 travel toward the first electrode 4 to which the voltage E1 is applied. While travelling, the secondary electrons 18 repeatedly collide with molecules of the gas in the sample chamber 1 and are thus gas-amplified. Then, the secondary electrons 18 flow into the first electrode 4. The signals generated by the gas-amplified secondary electrons 18 flowing into the first electrode 4, that is, the secondary electron signals of the sample picture are converted into the sample picture signal S1 in the form of a voltage by the sample picture signal amplifier 13.
According to the above-mentioned prior art, the sample picture signal S1 is obtained from the secondary electrons which repeatedly collide with molecules of gas to be gas-amplified. In this case, however, the signal-noise ratio of the obtained sample picture signal is not large enough, which is troublesome. Therefore, improvement of the signal-noise ratio of the sample picture signal is desirable.